Because of possible minor shifts between the frames of a film and its perforations, or to compensate for tolerances in the dimensions of a slide holder, it is often desirable to move the light beam of a projector in a direction parallel to the receiving surface. This may be achieved by transversely displacing the entire projection objective, as described in German utility model No. 7,208,770 or British Pat. No. 419,177. Another solution, described in U.S. Pat. No. 3,963,337, involves the use of a swivelable lamp housing containing a corrective lens assembly. A further system, described in German published specification No. 2,411,740, involves the shifting of a slide support with reference to a pair of associated projection objectives.